compact camera module testing equipment with a conversion lens · compact camera module testing...

Compact camera module testing equipment with

a conversion lens

Jui-Wen Pan* 1Institute of Photonic Systems, National Chiao Tung University, Tainan City 71150, Taiwan

2Biomedical Electronics Translational Research Center, National Chiao Tung University, Hsin-Chu City 30010, Taiwan

3Department of Medical Research, Chi Mei Medical Center, Tainan 71004, Taiwan

*[email protected]

• Abstract: Modified testing equipment for adjusting the back focal length

of a compact camera module (CCM) is proposed. The advantages of this

modified testing equipment which includes a conversion lens are that it

saves on testing space, offers a smaller sized testing chart, as well as high

speed chart changing, and variable object distances. The modified testing

equipment can produce a test chart of 38.32 mm compared to an equivalent

testing chart of 5000 mm with the conventional testing equipment. At the

regular object distance of 2000 mm, both total track and testing chart size

for the modified test equipment were 8.3% that of the conventional testing

equipment. By using this testing equipment, the testing space can be shrunk

significantly.

©2012 Optical Society of America

OCIS codes: (080.3620) Lens system design; (080.2740) Geometric optical design; (080.3630)

Lenses.

References and Links

1. C. S. Liu and P. D. Lin, “A miniaturized low-power VCM actuator for auto-focusing applications,” Opt. Express

16(4), 2533–2540 (2008). 2. C. S. Liu and P. D. Lin, “Miniaturized auto-focusing VCM actuator with zero holding current,” Opt. Express

17(12), 9754–9763 (2009).

3. N. Sugiura and S. Morita, “Variable-focus liquid-filled optical lens,” Appl. Opt. 32(22), 4181–4186 (1993).

4. J. W. Pan, “The first order optics of novel testing equipment for compact camera module,” IODC (OSA 2010)

paper JMB25 (2010).

5. W. J. Smith, “Stops, apertures, pupils and diffraction,” in Modern Optical Engineering, 4th ed. (Mc-Graw Hill, 2008), pp. 182–183.

6. Largan Precision Co, Ltd. “Conventional compact camera module specification,”

http://www.largan.com.tw/html/product/product_cellphone_en.aspe 7. W. J. Smith, “Scanner/f-θ, laser disk and collimator lenses,” in Modern Lens Design, 2nd ed. (Mc-Graw Hill,

2005), pp. 561–571.

8. M. Laikin, “F-theta scan lenses,” in Lens Design, 4th ed. (CRC Press, 2007), 245–252. 9. R. E. Fischer, B. Tadic-Galeb, and P. R. Yoder, “Hardware design issue,” in Optical System Design, 2nd ed.

(Mc-Graw Hill, 2008), pp. 594–595.

10. G. C. Holst, “Sampling theory,” in Electro-Optical Imaging System Performance, 4th ed. (SPIE, 2006), pp 72–87.

11. B. Hayashida, “F-theta lens system for four-group construction,” U.S. patent 4400063 (Aug 23, 1983).

12. K. Fuse, “F-theta lens,” U.S. patent 6324015 (Nov 27, 2001) 13. W. J. Smith, “Optical computation,” in Modern Optical Engineering, 3rd ed. (McGraw- Hill, 2000), pp. 301–

346.

14. M. J. Kidger, “Principles of lens design,” in Fundamental Optical Design (Mc-Graw Hill, 2001), pp. 158–159.

15. M. J. Kidger, “Principles of lens design,” in Fundamental Optical Design (Mc-Graw Hill, 2001), pp. 150–151.

16. J. M. Geary, “Lens splitting,” in Introduction to Lens Design with Practical ZEMAX Examples (Willmann-Bell,

2001). 17. R. E. Fischer, B. Tadic-Galeb, and P. R. Yoder, “Hardware design issue,” in Optical System Design, 2nd ed.

(Mc-Graw Hill, 2008), pp. 675–677.

18. Omni Vision Technologies, Inc., http://www.ovt.com/ 19. W. J. Smith, “Stops, the practice of optical engineering,” in Modern Optical Engineering, 4th ed. (Mc-Graw Hill,

2008), pp. 620–622.

#160731 - $15.00 USD Received 3 Jan 2012; revised 9 Feb 2012; accepted 9 Feb 2012; published 17 Feb 2012(C) 2012 OSA 27 February 2012 / Vol. 20, No. 5 / OPTICS EXPRESS 5303

20. IEEE resolution target chart, Spring Master Source Book (Edmund Optics, 2011), p. 363.

1. Introduction

Small sized portable electronic devices such as mobile phones and personal digital assistants

(PDAs) are becoming common consumer electronic devices especially for the purposes of

time management and information communication. An essential element of these consumer

electronic products is comprised of the imaging device. In order to include the imaging device

in these devices, the imaging and sensing components are packaged into a compact size

module called the compact camera module (CCM). The CCM consists of complementary

metal-oxide-semiconductor (CMOS) sensors, a lens holder and a pick-up lens. The

characteristics of the CCM such as its low cost, compact volume and simple structure, allow it

to be widely applied in web cameras and mobile phones. However, the CCM is simple in

structure, so lacks a dynamic focus compensation mechanism, such as voice coil motor

(VCM) and liquid lens [1–3]. Thus, the accuracy of alignment between the pick-up lens and

imaging sensor becomes an important issue in its assembly.

The current method for testing the CCM assembly is to use a real testing chart to adjust

the back focal length of CCM as a compensator. A testing chart is localized in front of the

pick-up lens and then the image quality on the CMOS sensor is used to judge whether it was

assembled on the back focal plane of the pick-up lens. If the back focal length between the

CMOS sensor and pick-up lens cannot be assembled precisely, the image quality will be

blurred which impacts the yield during CCM mass production. Generally, the object distance

for the CCM in a mobile phone can vary from 200 mm to 5000 mm depending on the

application. In order to find the correct back focal length in conventional testing equipment,

the testing chart is moved along the normal direction of the CCM for assembly quality

examination. Thus the volume of conventional testing equipment needed is large and testing

chart fabrication cost expensive. The volume of testing equipment needed can be reduced by

the insertion of a folding mirror in the imaging path which can reduce the horizontal space.

However, there is a simultaneous increase in the vertical space because the total track of the

optical path for an image pick-up system remains constant under the conjugation relations

between an object and an image. In addition, the folding mirror also increases the equipment

cost and induced alignment issues in the mass examination process. In other words, the

conventional testing equipment still has the problem of possessing an overly large volume, an

aspect which needs to be improved.

In this study, the enables conversion lens is needed in order to shrink the testing

equipment to meet the size limitations of 120 mm × 100 mm × 250 mm. The conversion lens

was inserted into the testing equipment to produce an erect virtual image. The novel

conversion lens testing equipment possessed a short testing period and a compact size which

is also helpful to improve the mass production of CCMs. Moreover, the image quality,

resolution and distortion are almost the same as for traditional testing systems.

2. Design concept and specifications for the conversion lens

2. A. First order optics

A conceptual representation of the novel CCM test equipment is shown in Fig. 1. The system

consists of a CCM, a conversion lens and a testing chart. The inclusion of the proposed

conversion lens allows a significant decrease in the volume of the testing system. The

conversion lens produces an erect virtual image which is used as the new testing chart for the

CCM module testing. The first order equations have been described in detail in a previous

publication [4]. According to the data, the conversion lens and CCM test lens have a first

order relationship, but the result is only a paraxial optics result. Moreover, the design

architecture is based on both telecentric lenses and f-tan(theta) lenses. Each feature offers

some advantages for CCM testing. First, there is an advantage for the lens architecture with


the telecentric conditions. It is widely utilized in optical systems because it tends to reduce the

measurement or position errors caused by some defocusing issues of the testing equipment

[5]. In other words, the magnification will be constant at full field not only under the paraxial

optical conditions, when moving the testing chart. Second, the field of view (FOV) of CCM

systems is typically around 60-65 degrees [6]. In order to match the FOV of the CCM and

conversion lens, the FOV of the conversion lens should be equal to the FOV of the CCM.

Then, the main task is to use the f-tan(theta) lens architecture to produce the image in the

CCM. In other words, the conversion lenses have two features, the f-tan(theta) lens and

telecentric conditions for novel testing application. We can also call this conversion lens a

telecentric f-tan(theta) lens.

Fig. 1. Conceptual representation of the novel CCM test equipment layout.

There are a lot of applications which use the f-tan(theta) lens architecture such as in

LASER scanning systems [7, 8]. The paraxial conditions for the f-tan(theta) lens are outlined

in the following equation:

tan( ),2

conversion

conversionh f

(1)

where h is the image height of the test chart; fconversion is the focal length of the conversion

lens; θconversion is the FOV of the conversion lens. Owing to the f-tan(theta) lens translating the

chart image into the CCM, the FOV of the conversion lens must be equal to the FOV of the

CCM at least. By using Eq. (1), we can design the correct image height for the testing chart

for different FOVs for CCM testing. Moreover, the magnification of the novel test equipment

for infinite virtual images produced by the conversion lens [4] is obtained by

tan( )2 ,

tan( )2

conversionconversion

conversion

ccm ccmccm

ff

Mf

f

(2)

where fccm and θccm are the focal length and FOV for the CCM lens; M is the magnification of

the novel CCM test equipment. According to Eq. (2) and a previous study [4], the best

working condition for a conversion lens is when the virtual image is at infinity for the purpose

of saving space during testing.


2. B. Conversion lens specifications

The conversion lens and pickup lens in the CCM test equipment are independent optical

elements. For assembling the two optical elements into one system, pupil matching is very

important [9]. The diameter of the entrance pupil of the CCM must be smaller than that of

conversion lens in order to satisfy alignment tolerance issues during pupil matching. The

basic architecture is that of an f-tan(theta) lens. In the entrance pupil, typically called the

external entrance pupil, there is some distance between the lens groups, so there is free space

to match the pupil between the CCM and the conversion lens. The diameter of the lens group

has a relationship to the FOV of the conversion lens and the distance between the exit pupil

and lens group of the conversion lens [7, 8]. In order to reduce the size of the conversion lens

and make alignment easy, the distance is 10 mm in this conversion lens design. Moreover, the

diameter of the entrance pupil of the conversion lens is always larger than the entrance pupil

diameter of the pickup lens of the CCM in order to ensure alignment tolerance. According to

CCM specifications, the typical effective focal length (EFL) for a CCM is around 2.7-4.5 mm

at a working F/# between 2.0 – 3.2 [4, 6]. So the typical entrance pupil diameter (EPD) for a

conversion lens would be 3 mm to cover all CCM lenses. The typical FOV of a CCM is

around 50-65 degrees [6]. Using a conversion lens with an FOV of 65 degrees, this could

cover all FOVs for CCMs in the market.

For the f-tan(theta) lens, the exit pupil is the stop size. For such a design, the stop size is 3

mm to ensure pupil matching and tolerance consideration. Moreover, the focal length of the

conversion lens is an important parameter for optical specifications. According to Eq. (2), the

focal length during conversion is relative to the magnification of the conversion lens and the

CCM. In order to reduce the size of the system, the magnification should be as small as

possible. The focal length of the conversion lens will be smaller for a compact system. Owing

to a smaller focal length of conversion lens, the F/# of the conversion lens will get smaller but

with higher aberration under the same EPD. To satisfy the purpose of the testing system, the

conversion lens should introduce as little aberration as possible. So the focal length cannot be

as small as it could possibly be. Another issue is that the pattern chart on the image plane of

this conversion lens must be a constant picture with analog signs for avoiding the moiré issue

at the CMOS sensor [10]. In order to the moiré-free issue, the best pattern chart should be

camera film which has a typical width of 6 cm (60 mm). For the typical sensor the aspect ratio

is 0.75, the diagonal diameter of the patent chart film is 10 cm. Besides the pupil matching,

the field angle of a CCM must be equal to the field angle of the conversion lens to ensure

imaging translation. Based on Eq. (1), the focal length of the conversion lens is found to be 78

mm. According to the above, we can get F/# of 26 for the conversion lens. Finally, all

information specifications for the conversion lens are shown in Table 1.

Table 1. Specifications of Conversion Lens

Item Parameters

Architecture telecentric f-tan(theta) lens

Effect focal length (EFL) 78 mm

Field of View (FOV) 65 degrees

Entrance Pupil Diameter 3mm

F number 26

3. Optical design, tolerance and system verification

3. A. Optical design and the layout of the conversion lens

Based on the specifications in Table 1, we searched the United States Patent database for

results for the starting point of the conversion lens. According to the search results, US patent

4400063 and US patent 6324015 were used for the starting point of our design [11, 12].

Owing to the inherent properties of an f-tan(theta) lens with telecentric conditions, the lens

not only has a complex structure, but also the diameter of the last lens group will be larger


than the image size. The final lens design layout after optimization is shown in Fig. 2. All lens

elements in the conversion lens design are positive lenses so have a logical configuration of

(++++).

17:56:55

Fig. 2. Optical layout of the conversion lens design.

According to this conversion lens design, different virtual image distances can be

produced for different real object distances [4]. Figure 3 shows the relationship between the

virtual image distance and the real object distance for the conversion lens. A real object

distance of 38.32 mm can produce a virtual image distance of 5000 mm using this final

design. This phenomenon confirms that this new test equipment with the conversion lens can

significantly reduce the testing space.

0 1000 2000 3000 4000 5000

24

26

28

30

32

34

36

38

40

Rea

l O

bje

ct d

ista

nt(

mm

)

Vertial image distant(mm)

Fig. 3. Relationship between real object distance and virtual image distance for the conversion lens design.

3. B. Optical performance of the conversion lens

3. B. 1.Seidel aberration

According to lens specifications, the F/# of the conversion lens should remain at a high

number and the Seidel aberration beside the optical distortion should be held at a lower level

[13]. The optical distortion is a shape-dependent aberration. The lens shape and lens bending

factor are used to control the optical distortion at the lower level, as shown in Fig. 4 [14].


Moreover, the field curvature is relative to the Petzval sum. For the flat imaging sensor

applications, the Petzval sum is an important Seidel aberration. The basic way to reduce the

Petzval sum is to use separate lenses with a high index material. In this design, the total lens

power is divided into four positive lenses with high index material arranged in the (++++)

configuration. This technique is often used in lens design to reduce the Petzval sum [15, 16].

In addition to the Petzval sum, lens splitting is also helpful to reduce the occurrence of

spherical aberration in the conversion lens [14]. Since the color aberration is a shape-

independent aberration, we only use an achromatic doublet lens as the second lens in the

conversion lens design to reduce the color aberration in the conversion lens. In short, all

aberrations are well controlled for the high F/# conversion lens.

14:36:43

ASTIGMATIC

FIELD CURVES

ANGLE(deg)ST

36.00

28.59

19.96

10.29

-5.0 -2.5 0.0 2.5 5.0

FOCUS (MILLIMETERS)

DISTORTION

ANGLE(deg)

36.00

28.59

19.96

10.29

-2 -1 0 1 2

% DISTORTION

23-Mar-11

Fig. 4. Field curvature and optical distortion of the conversion lens.

3. B. 2. Modulation Transform Function (MTF) of the conversion lens

For the CCM system, the pixel size is the key parameter relative to the spatial frequency. In

order to evaluate the spatial frequency of the conversion lens, the magnification is the key

item needed to calculate the different Nyquist frequencies corresponding to the different pixel

sizes at the conversion lens with an infinite virtual image [17]. The following table shows the

relationship used in the conversion lens testing CCM with a typical EFL of 3mm.

Table 2. Pixel Size at CCM Sensor and Image Pixel Size after Passing through

Conversion Lens (Typical EFL of CCM is 3mm and EFL of Conversion Lens is 78mm)

Pixel size (μm) at CCM with

Omni Vision sensor [18]

Pixel size (μm) at

conversion lens

Corresponding Nyquist frequency

(lp/mm) at conversion lens

1.75 45.5 10.9

3.0 78 6.4

4.2 109.2 4.5

6.0 156 3.2

Based on Table 2, the modulation transform function (MTF) performance of the

conversion lens with infinite virtual images is shown in Fig. 5. The MTF values at the Nyquist

frequency for different pixel sizes are larger than 0.5 for all field heights. For the typical CCM

module, the design frequency is half of the Nyquist frequency and the testing frequency is one

third of the Nyquist frequency. This confirms that the conversion lens does not change the

resolution performance of the testing CCM. In terms of image quality, the only limitation of

the conversion lens is when the virtual image is at a short distance. The MTF value obtained

when the virtual image distance changes to 300 mm is shown in Fig. 5(b). The minimum

required MTF for a clear image is 0.3. At a field height of 0.8 to 1.0 the MTF height is below


0.3 when the spatial frequency is more than 4.6lp/mm. So the quality of the test image should

be affected by the conversion lens. There should be a limitation for the test results obtained by

using the conversion lens for virtual images distant of less than 300 mm.

Fig. 5. MTF diagraph of the converstion lens for an virtual image distance of (a) infinity and

(b) 300 mm.

4. Tolerance analysis

The last step of lens design is tolerance analysis. The key for mass production is the level of

precision of the tolerance data [19]. The precision tolerance data shown as Table 3 is used in

tolerance analysis. This is found by utilizing the MTF tolerance function in the Code V

software to verify the production ability. The results are shown in Fig. 6. According to the

tolerance results, all parameters for the conversion lens design are precise enough to be

developed for mass production under the precision level [19]. Even though the cost of the

precision level is higher than the cost of the commercial level, the conversion lens design is

used only in testing equipment with low quantity. Moreover, under cumulative tolerance, the

MTF value for the 1.0 field is 0.43 at a Nyquist frequency of 10.9 lp/mm. The MTF value is


typically larger than 0.3 for a clear image. Of all parameters, the most sensitive is the element

tilt and wedge for the first element. The main reason is that the incident angle of this lens has

the highest value in this conversion lens design.

Table 3. Tabulation of Precision Optical Fabrication Tolerances

Items Parameters

Surface Quality 60-40 Diameter (mm) ± 0.02

Deviation (concentricity), (min) 1-3 Thickness (mm) ± 0.1

Radius (Fr) 5 Regularity (Fr) 1

Linear Dimension (mm) ± 0.1 Angles ± 5-10’

Fig. 6. MTF value Tolerance diagaph.

5. System verification

The finished conversion lens is shown in Fig. 7. In order to verify the functioning of this

conversion lens, we used CCM lens to verify the novel testing equipment by IEEE resolution

target chart in a conversion lens [20]. The name of the model CCM lens is Largan 9312C with

a 2M pixel sensor [6]. The relationship between the virtual image and the real object is shown

in Fig. 3. The conversion lens produced a virtual image of 2000 mm from a real object at a

distance of 37.14 mm. The CCM lens captured a clear virtual image produced by the IEEE

resolution target chart of the conversion lens. Images of the IEEE resolution target chart

(center and corner) obtained with the conversion lens are shown in Figs. 8 (a) and 8 (c). The

same CCM captured the real IEEE resolution target chart at 2000 mm without a conversion

lens. Images of the IEEE resolution target chart (center and corner) obtained without a

conversion lens are shown in Fig. 8(b) and 8(d). The resolution is the same at the center of the

image at the 600 TV line, with and without a conversion lens. The resolution of the corner

image at the 600 TV line is also the same as the center image. From this we can confirm that

the test equipment with the conversion lens had no impact on the resolution. Moreover, the

total tracking was shrunk from 2150 mm to 180 mm by the conversion lens. The testing space

was reduced down to 83/1000. In addition to this, the telecentric conditions for the testing

chart in the conversion lens design will maintain an almost constant magnification for a long

virtual image distance range. This phenomenon will perform the through focus MTF testing


without changing the testing chart. So this novel test equipment could be utilized for many

tests for different vertical object distances at a constant spatial frequency without changing the

testing chart.

Fig. 7. Photograph of the conversion lens sample.

Fig. 8. (a) Shows the center (a) and corner (c) CCM images obtained with the conversion lens,

and the center (b) and corner (d) CCM images obtained without the conversion lens.

6. Conclusion

In this study, a set of modified testing equipment was developed by the insertion of a

conversion lens into the conventional testing equipment for CCM examination. The modified


testing equipment possessed several advantages over the conventional testing equipment such

as being simple in structure, compact, having a fast test rate, and cost reduction. The modified

testing equipment was quite simple in structure requiring the insertion of only one conversion

lens. The virtual image produced by the conversion lens was compact in volume. This virtual

image was adopted as the new object in the modified testing equipment. The total track of this

modified testing equipment was 83/1000 of the conventional testing equipment at an object

distance of 2000 mm. The image quality obtained using the new testing equipment was also

examined. The conversion lens should not have any effect on image resolution under high

precision tolerance. By using this technology, the cost and space for the novel testing

equipment can be significantly reduced.

Acknowledgments

This study was supported in part by the National Science Council, project number NSC100-

2220-E-009-023 and NSC101-2314-B-384-001, in part by the Chi Mei Medical Center, project

number 100C022 and in part by the "Aim for the Top University Plan" of the National Chiao

Tung University and Ministry of Education, Taiwan.


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compact camera module testing equipment with a conversion lens · compact camera module testing...

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